Inasmuch as the storage capacity of sources of electric power is not large at best electrodes for the purpose of devices such as those of the invention are such as to be economical as to the current drained. One of the possible ways of developing such electrodes is to reduce the ohmic resistance thereof. This can be attained first by selecting a material having low ohmic resistance and secondly due to an appropriately provided electrode construction.
Known in the present state of the art are electrodes whose current supplying lead is made of materials having low ohmic resistance (cf., e.g., "Engineering Analysis of Pacemaker Electrodes" by Daniel W. van Heeckeren and James F. Hogan, Annals of New York Academy of Sciences, 1969, v. 167, p.p. 774-784). Pertaining to such materials are, for example, the alloys of the following weight percent composition:
______________________________________ platinum 90.0 iridium 10.0 (I) as well as palladium 35.0 silver 30.0 copper 14.0 (II) platinum 10.0 gold 10.0 zinc 1.0 ______________________________________
In addition, used as such a material is stainless steel clad with gold and platinum.
However, all these materials incorporate highly expensive rare metals and feature low mechanical fatigue as compared to an alloy extensively applied for the purpose currently; said alloy is based on cobalt and has the following weight percent composition:
______________________________________ cobalt 40.0 chromium 20.0 nickel 15.0 molybdenum 7.0 manganese 2.0 (III) carbon 0.15 beryllium 0.04 iron 15.81, ______________________________________ the above alloy though being inferior to the afore-mentioned ones as to ohmic resistance has much higher endurance. Thus, electrodes made of the alloy (III), under otherwise equal operating conditions, prove to be 330 times more durable than electrodes made of the alloy (I).
Some electrodes are known to be in current use, wherein a reduced ohmic resistance is achieved due to appropriately selected construction of the electrode.
Thus, e.g., 49-conductor braided stainless electrodes are available from "General Electric Co.", while the firm "Electric" produces 49-conductor braided electrodes made of gold- and platinum-clad stainless steel. However, the former electrodes are 25 times and the latter, 5 times less durable than the electrodes whose current-supply lead is a coil winding featuring its coils wound closely to one another (cf., e.g., the afore-mentioned article "Engineering Analysis of Pacemaker Electrodes").
Some attempts are known to have been made to provide a current-supply electrode lead as a few (e.g., four) parallel wound coil springs (multiple-coil winding) as this substantially reduces the ohmic resistance of a current conductor (cf., e.g., W. Irnich "Engineering concepts of pacemaker electrodes", "Engineering in Medicine", 1, "Pacemaker Technology", Berlin-Heidelberg-New York, 1975).
The author claims that such a construction is not only instrumental in reducing the ohmic resistance of the supply lead (which proves to be an indisputable fact) but also provides for an adequate flexibility and an increased endurance of the electrode. The author explains the fact by that all the four coil springs would have to break before the electrode fails. It is quite evident, however, that a maximum flexibility is attained (with other things being equal) when the coils are wound as closely as possible to one another, i.e., when the winding pitch equals the diameter of the wire conductor being wound (i.e., t.apprxeq.d), which is attainable in the case of a single-layer winding. A two- or multi-layer winding will affect adversely the flexibility of the electrode. A multi-layer winding augments the coil-to-coil spacing (or winding pitch) multiply, thus approximating the coil rod to the straight elastic bar and thereby adding very much to the flexural rigidity of the conductor, which reduces considerably the endurance fatigue of the coils of any of several windings. It ensues from all the discussed above that failure of any separate coil occurs much earlier than in the case of a single-layer winding having the coils spaced as close as possible to one another. If only a single coil fails its sharp and rough edges will rub against the other coils, thus causing an intense fatigue wear and breakdown of the latter.
The problem of devising extensible electrodes is far from being solved at present altogether. Practically the problem is being solved due to diverse surgical techniques and approaches, such as the provision of semiloop-like (loosely hanging) excess conductor lengths inside and close to the organ, as well as along the transvenous route and at the place of connection of the distal electrode head to the source of power supply, or by replacing the electrode after a definite lapse of time (in cases where the electrode has been implanted in a growing organism).
However, such approaches can solve the problem but incompletely as the provision of semiloop-like excess conductor lengths is not feasible in all cases (e.g., in the case of small-diameter veins) and is of low efficacy as the electrode body is liable to get enveloped in fat and other cells and becomes of low mobility as to length direction. On the other hand periodic replacement of the electrodes requires further expenses for a corresponding operation and fresh electrodes.
Another electrode for connecting to an internal organ of human body is known to comprise a body accomodating a supply lead shaped as a coil spring rigidly coupled to the distal and contact heads thereof which are located at the ends of said body, the coils of the spring on the faces thereof are spaced as close as possible to one another (cf., e.g., model 6905, 6907 endocardial monopolar electrodes available from "Medtronic Co.", USA).
In the latter electrode the coils of a coil spring are arranged in a nearest proximity to one another throughout the spring length, i.e., with the same pitch along the entire length thereof. The electrode body is made as a plain-walled tube from a dielectric material.
Such an electrode features high ohmic resistance of the wire lead thereof which results in a high-rate exhaustion of the source of electric power (such as the battery of a pulse generator) and results in a higher threshold of electric stimulation of the organ involved. The electrode requires extensive consumption of precious metals (platinum, silver) from which the supply lead of a majority of electrode modifications is made. In addition, the electrode is inadequately extensible lengthwise in the course of operation which might result later in its lengthwise displacement and separation of the electrode contact head from the place of contact with the organ involved (e.g., several years after the implanting), which is likely to occur most commonly with the implanting is in a growing organism, or as a result of apparent changes in the patient's stature or of abrupt motions of the patient e.g., during sports games, while dancing or as a result of falls. In all these cases the contact head of the electrode is liable to displace, whereby an ineffective stimulation occurs in some attitudes of the patient's body.